Timing modulation



May 21, 1946. N. l. KoRMAN TIMING MODULATION 2 Sheets-Sheet l Filed June30, 1943 ATTORNEY May 2l, 1946. N. n. KoRMAN TIMING MODULATION FiledJune 30, 1945 2 Sheets-Sheet 2 INVENTOR MNM/wa 1PA/,4m

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ATTORNEY up wave is multiplied in frequency,

. rier controlled 'oscillations nor the beating oscilla-l land all ofthe other 'ating voltages Patented May 21,l 1946 TIMING MoDULA'rIoN.

Nathaniel I. Korman, Camden, N. J., assigner to y Radio CorporationofAmerlca, a corporation oi Delaware Application June 30, 1943, SerialNo. 492,841'

1s claims. (01. 17o- 1715) A requirement in a. frequency or phasemodulator is that the mean carrier stay Within its assigned place in thefrequency spectrum. The Federal Communication Commission requires thatthe carrier frequency must be maintained within prescribed limits, thepresent limits requiring a stabilityof +2000 C. P. S. in the 42-50 mc.band or better.

Two frequency modulation systems have attracted considerable attentionin the radio art. In one of'these systems a crystal controlledoscillator supplies a carrier which is modulated in amplitude inaccordance with modified signals,

the side bands are selected and recombined with,

the carrier which has been shifted 90 in Phase. This resulting built upwave has a Lzharacteristic of a wave which has been frequency modulated.The carrier frequency stability is that of the crystal oscillator but inorder to have low harmonic distortion a small phase variation only maybe obtained in the built up wave. In order to obtain the wider deviationdesirable this built six thousand times and the multiplication steps arebroken up by heterodyning down steps.

This sort of system is obviously complicated and uses a large amount ofapparatus.v Moreover, an'ects the frequency lstability of the nalcarbecause' it is no longer that of the crystal tions. It is amodulation product of the several oscillations. The stability of thecrystal confrequency thereby necessitating less multiplication and.resulting in simplication' of the entire arrangement. Bycarefulattention to the frequency discriminator circuit used to derive thecontrol potentials the carrier frequency can readily be maintainedwithin the assigned limits. However, in any system where a largefrequency deviation is used, the modulated wave consists of a largenumber of side bands of considerable energy content and a carrier'ofrelatively small energy content. This renders more difficult selectionand linear conversion of the saine by the discriminator (slopingcircuit) to obtain control v potentials truly characteristic of carrierdrift. If the frequency deviation or excursions are reduced to a largedegree, say for example, to a degree less than the lowest signalfrequency, the modulation then consists of a strong carrier or meanfrequency component and a few upper and say five orl the use of thebeating down oscillations trolled oscillations and the beatingoscillations factors including temperature variations,

in the problem, thereby rendering it more difllcult to obtain thestability required.

In the second method or system which was devised M.4 G. Crosby (CrosbyPatent #2,279,659, dated April 14, 1942) direct timing modulation isused.. A' generator of the selfgeneration Y type, has its frequencymodulated directly by a reactance tube and its mean frequency isstabilized by selecting some of its output, subjecting the same todiscrimination and rectication and using the output obtained byrectication to 'regulate the blason a control electrode of the reactancetube in la sense to oifset or nullify tendencies of the oscillatorfrequency to drift from its assigned frequency.

This second method and means has been widely accepted and has numerousadvantages. The initially modulated oscillations may be of higher vacuumtube dissimilarity, operj and other circuit elements enter lower sidebands of relatively negligible ampli.

tude. It has been suggested that this reduction of deviation beaccomplished by frequency division. This, however, entails a largenumber of frequency divider stages and again the system becomes unwieldydue to the large number of divider stages and apparatus used with thesame. vIn my present system direct frequency modulation, as illustratedin said Crosby patent, is used, and in my new improved means and method,to reduce the deviation to derive substantially mainly carrier energywith reduced modulation components for control purposes, I make use ofthe principle of degeneration.

An object of my invention then is to stabilize the frequency ofgenerated oscillations.

A particular object of my invention is to stabilize the mean frequencyof generated oscillations the timing of which is modulated extensivelyin accordance with signals.

The above objects are attained in accordance with my invention bydiverting wave energy from the generator, subjecting the diverted energyto degeneration to `an extent such that frequency or phase deviationsthereof are reduced to a low 's value such that they may be subjected tophasedetection and the demodulation components used to control the meanfrequency ofthe generator in a sense to Wipe out undesired drifts orvariations in its meanlfrequency.

Details of the manner in which the above objects arc attained, as wellas other objects and the manner o f attaining the same, will appear fromthe detailed description which follows. In this detailed descriptionreference willlbe made to the attached drawings wherein Figs. 1 and 2 2each illustrates schematically and mainly Aby block diagram, embodimentsof timing modulation systems arranged in accordance with my invention,while Fig. 3 illustrates details of the phase detector used in theAmodification of Fig. 2.

I is a self-,excited oscillation generator designated hereinafter asoscillator #1. 'I'his oscillator generator is modulatedl as to itstiming by signals fed to the modulation input of a modu- Vlatorl I4coupled to the oscillation 'generator I0. This modulator may be of anyapproved type and herein is assumed to be of the reactance tube typesimilar to that shown, for example, in Crosby Patent #2,279,659, datedApril 14, 1942.

The timing modulated oscillations are supplied to an output and also toa frequencyV converter I6 designated hereinafter as converter #1; In

, this converter, which may be of any well known type, the timingmodulated oscillations are heterodyned with oscillations of constantfrequency derived from an oscillator I8 which may be of the crystalcontrolled type.

The timing modulated oscillations are reduced in frequency in converterI6 and supplied to a converter designated hereinafter as-converter #2.In this converter oscillations of a reduced frequency are mixed withoscillations from a selfexcited oscillation generator 28, designatedhereinafter as self-excited oscillator #2.

The output of converter #2, which maybe the sum or difference frequency,is supplied to a fre-- quency modulation detector 24 wherein the tim- Ying modulations on the wave energy are detected sothat at the outputfrom 24 is derived potential variations characteristic of the timingmodula- Ation and of the slow changes or drifts in the" mean frequencyof the intermediate frequency out of the converter in 20. This frequencymodulation detector may be of any approved type, such as, ./for example,Vas shown in Crosby Patent #2,279,659, dated April 14', 1942, or inSeeley Patent #2,121,103, dated June 21, 193s. Y The output selectedfrom 24 is fed through a lter 25, wherein the direct current is removed,and acts through a reactance tube modulator such as, for example, usedat I4 to control the oscillator in 28. Units 2l, 24, 2G and 22 comprisea degenerative loop wherein, as will be set forth morein detailhereinafter, the modu-- lations on the wave energy are reducedconsiderably such as, for example, to a phase amplitude of less thanone-radian. This degenerative loop utilizes the principle involved inHansel! Patent #2,205,762, dated June 25,1940, and Crosby Patemiy#2,191,518, dated April 1e, 1940. 5

Intheoutputofconvertei-#2duetothisdegeneration the frequency of thecarrier is substantially unchanged while the swing is' reduced toasmallphase amplitude andtheoutputisfedtoaphasedetectorwhichalsoissupplied'with oscillations from stableoscillator il, the output frequency of which is the` same as the averagefrequency of the output of converter #2. A network 5l is providedbetween the output of the phase detector andthe modulator to filter loutall audio frequency currents.

Thephasedctectormaybeofanyapproved type and. as illustrated, utilm theprinciple involved 1n crosbyratenr #2,229,610, dated Jan-f nary 28,1941.-

the phase detector it will be notedthattheoutputofconverterwisimpressedbyatransformerlldiiferentiallyonapairof diodes I2 and 44, while theoscillations of con-YL etant frequency from Il ade-impressed in parallelY controls the reactance tube in 26 to modulate the output from 20 sothat the resultants on the A anodes change differentially in vamplitudeas do the potential drops in resistance and condenser units 46 and 48.This producesacross 46 and' 48 a potential which represents the changesin frequency of the carrier in Ill. The demodulated components arefiltered by resistance condenser units 46 and 48 and, if necessary, byadditional' filtering circuits in 50 and supplied tothe react- 1 ancetube modulator I4 in a sense totend to oppose undesired changes in themean frequency yof the oscillations generated in I0.

y The operation ofthe timing modulation system including self-excitedoscillator #l and modulator I4 is well known in the art, and isdescribed in the CrosbyPatent #2,279,659, referred to above.

Some of the timing modulated output energy is fed to I6 andheterodynedwith oscillations from crystal oscillator I8 and thedifference .frequency is selected inv converter #l a'nd fed to converter#2. The difference frequency is taken here in preference to the sumfrequency in order that the self-excited oscillator #2 may operate at alow frequency and a relatively'low frequency output may be derived fromconverter #2. The second intermediate frequency output of converter #2may beat the sum or difference frequency and this output is fed to thephasefdetector 38 and tothe frequency detector 24. The

detected modulation out of the detector in 24,

self-excited oscillator #2 in a degenerative sense, so that when Atheoutput of 28 is mixed with the output of IB in converter #2 the timingmodulation is reduced to an extent such that it isv rep# resented by notmore than one radian phase l amplitude. vWhen the sum intermediatefrequency is selected at the output of converter 2U the self-excitedoscillator #2 is to be modulated in 'such a direction thaty themodulationenvelope thereof is'opposed in phase with respectA to themodulation envelope on the energy atthe output of converter #l in I6. Bythe expression modulation envelope used here I use the term somewhat inthe same manner in which it is used in connection with intensitymodulation.`

When the difference frequency is selected at the output of converter 20,the self-excited oscillator #2 in 28 is timing modulated in such a sensethat the modulation envelope 0f the output thereof is in phase with themodulation envelope of theenergy in the output of the converter #1', i.e., the frequency deviations are in the same direction instead ofopposed, 'as -is the case when the sum frequency is selected fromconverter #2.

polarity of the control DOtentials The proper supplied to the reactancetube modulator in 26 to obtain the desired sense f control of the self-yexcited oscillator #2 in 28 to make the loop degenerave, may beobtained-in various manners. For example, reversal of the detector 24output leads reverses the polarity ofthe control potentials. The use ofan additional modulation ain-f pliiler coupling stage between detectorand 26 will also reverse the polarity of thecontrol potentials; In thisrespect, it is noted thatA if the ^control potentialsl are ofinsuillcient amplitude to v accomplish the desired degeneration toobtain not more vthan one phase swing in the output anodes y ofconverter #2, an amplifier may be included in the regenerative loop andthis amplifier may be at the output of detector 24 and serve the doublepurpose of correcting the control potential polarity and increasing itsamplitude the required amount. The output of converter #2 of reducedphase swing is then fed to phase detector 88,*as is output from sourceI8 and the detected. components are filtered and used to control themoduvl quency for the #l converter in I8, and the delator .I4 to tunetheoscillator V#1.

In the modification of Fig. 2, the phase detector 38" is coupled toconverter #l in .I 8, to carrier source I8 and also to oscillator #2 in28. The output of the phase detector- 88' is coupled through the audiofilter 50 to the reactance tube I' modulator in I4 and through thedirect current filter to the modulator in 26.

The phase detector 38' in the embodiment of Fig. 2, performs thefunctions of the converter #2 in I 8, the detector 24.and the phasedetector 88 of Fig.. 1. As shown in Fig. 3, a phase detector somewhat asused in Fig. 1, 'modified to meet the requirements of Fig. 2, issatisfactory. The in5- put supplied .at the terminals marked Il in Figs.

2 and 3, from oscillator #2, isfapplied dinerentially to the diodes 42and 44 by transformer 40'.

This. input is mixed in 38' withthe input Ifrom converter #1 applied inparallel to the diodes by transformer 45. Moreover, this combination ormixing is in a degenerative sense and the polarity of control ofmodulator 26 is made such by steps described above to insuredegeneration in thev loop. 'I'he sum or difference frequency, sayvthesum 1.01 mc., plus the radian or less phase modu- 'lation resulting fromthis mixing in the degenerative sense, is also detected in 88'.

'I'he output from I8 is also supplied at the leads ings appropriatefrequencies for the various waves A used in the various parts of thecircuit.

It is noted that the crystal oscillator I8 supplies to the converter #1in unit IB a 4.04 mega- 6 cycle wave and to the phase detectors I8 and88! a 1.01 megacycle wave. This unit I8 accordingly includes in additionto a crystal oscillator the-l necessary frequency multipliers ordividers to sup. ply appropriate outputs'to obtain the desired fresiredfrequency for 'the phase detectors 88;and 88.

I claim 1. 'I'he method of generating oscillations the timing of whichis varied in accordance with signais, and of compensating variationsinthe mean frequency of the timing'modulated oscillations to obtain timingmodulatedoscillations of substantially constant mean frequency whichincludes I these steps, heterodyning oscillatory energy characteristicof said timing modulated generated oscillations with otheroscillatoryenergy character- `istie of said timing modulated generated oscillationsin a degenerative sense to derive'resultant v energy the timing of whichvaries within small limits in a manner corresponding to Avariations .inthe timing of said generated oscillations, .fle-'l tecting saidresultant energy and varying the mean frequency of said generatedoscillations in a sense to opposeand nullify said first variations .in

a0 the mean frequency thereof. n

2. The methodv of generating oscillations the -freluency of which isvaried in accordance with signals., and of compensating variations inthe i mean frequency of the frequency modulated 04s--l 35. cillations toobtain frequency modulated oscilla'- tions of substantially constantmean frequency` which includes these Stepaheterodyning oscillatoryenergy characteristic of said frequency modulated generated oscillationswith other os'- 40 cillatory energy characteristic of .said frequencymodulated generated oscillations in a degenerative sense to' deriveresultant energy the fre,-

lquency of which varies withinabout 1 radian in marked 3 in Figs. 2 and3, and fed in parallel'by transformer to the `diodes 42 and 44. lThephase detectorv 88' therefore, in addition to mix- ./.ing the outputs ofconverter #l and oscillator #2y in a degenerative sense and detectingthe resulting current'of reduced phase swing, also detect the phaserelation of the demodulation compoy 4s quency of said generated`oscillations. detecting nents so that the direct current components inmodulator I4, as well as the audio components used in modulator 26,appear in the output leads,

marked 4 in Figs. 2 and 3. The diodes 42 and 44, which are non-linearcircuit elements, form all possible beats lbetween the various signalsimpressed on them: All the beats with frequencies greater than audiofrequencies are bypassed by the capacitors of resistance andcondenserunits 46 and 48.. 'I'he net result is that output 4 .con-

tains only the audio corresponding to the dierin I0 relative to thosegenerated in I8.. The oono densers 41 and 48 are to tune to resonance.the coils across which they are connected. This is not essential but ishelpful in obtaining good eiliciency.

Referring again to Fig. l, it is obvious that the output of the phasedetector 88 contains audio as well as direct current. This audio may beused to feed the circuits in 25, thereby dispensing with the need of thefrequency detector in l24. In addition, as explained above, the #2converter in unit 120 and the phase detector 88 may be comblned asshown-in Figs. 2 and 3.

Although I do not propose to limit my system to waves of particularfrequencies, I have, merely i v for purposes of example, indicated onthe drawA 1s .varied oscillations to obtain oscillations'of sub. -l

a manner corresponding to variationsin the fresaid phase variationsandvarying .the mean fre- .y quency of said generated `oscillations in asense. I

to opposev and nullify said rst variations in the meanfrequencythereof.3. The method of kgenerating oscillationsthe phase or frequency ofwhichL is Varied extensively'l in accordance with signals and ofcompensating variations in the mean frequency of the phase or frequencyvaried oscillations to obtain oscillations of substantially constantmean frequency which includes these steps, heterodyningoscillatoryenergy characteristic of said phase or frequency variedgenerated oscillations with other 'oscillatory energy characteristic ofsaid phase or frequency varied generated oscillations in a degenerativesense to derive resultant energy the phase of'which `varies within thelimits of about A one radian' in a manner corresponding to variations inthe phase or frequency of said generated oscillations, detecting saidphase variationsv of said. resultant energy and varying the meanfrequency of said 'generated oscillations in accordv y ance with saiddetected phase variations and in a sense to oppose and nullify saidfirst mentioned 7o variations in the mean frequency thereof. v 4. Themethod. of generatingoscillations the.

frequency of which is modulated extensively in n accordance with signalsand of7 compensating var iations in the mean frequency of the frequencystantiall'y constant mean frequency which includes these steps.heterodyning oscillatory energy characteristic of said frequencymodulated gena timing modulation detector, for deriving the modulation,a secondv generator modulated by the output of the detector and a secondconverter eratedoscillations with other oscillatory energy icharacteristic of said frequency vmodulated generated oscillations in adegenerative sense to de. rive resultant energy thev phase of whichvaries within the limits of one radian in a manner corresponding to'variations in the frequency of said generated oscillations, beatingsaid derived phase varied resultant energy lagainst oscillations of axed frequency which is the same as'themean frequency of said resultantenergy to-detect saidv phase variations, and varying'the mean frequencyof said generated oscillations in accordfor heterodyning said lowfrequency oscillations against the output of said second generator in adegenerative sense, a timing modulation detector coupled, to said loopand responsive to said selected energy and a control circuit couplingsaid timing modulation detector to said first generator forcontrolling'its mean frequency of operation in a sense to nullifytendencies of the mean frequency of operation thereof to drift.

.9. In apparatus of the class described, a frequency modulatedoscillation generator the mean ance with said detected phase variationsand in a sense to oppose and .i nullify'said first.. mentionedvariations in the mean frequency thereof.

5'. In a timing modulation system comprising a self-generating generatorand a modulator for modulating the timing of the generated oscillationsinvaccordance with signals, a mean fre' A .quency stabilizing circuitfor said generator ins cluding a degenerative loop to which saidmodulated energy is fed and from which energy comprising primarilycarrier energy is g selected, a timing modulation detector coupledtosaidloop trolcircuit couplin-g said detector to said generator forcontrolling its mean frequency of op eration in a sense to nullifytendencies of the mean frequency of operation thereof to drift.

6. In a timing modulation system comprising a 4and responsive tosaidselected energy and a confrequency of operation of which maychange,` a source of oscillations of fixed frequency and a firstconverter for derivingcorresponding modulated and changing oscillations,means for com-- bining said derivedyoscillations and said frequencymodulated oscillation in degenerative sense to obtain oscillatory energywherein the modulations have been reduced to" phase librations of theorder of one radian, a phase detector coupled to ,said last named means,and a control circuit coupling said phase detector` to said firstgenerator to change itsmean frequency of operation in a sense toAnullify said changes in its frequency of operation.

10. In apparatus of the class described, av frequency modulatedoscillation generator the mean frequencyof. operation of ,which maychange,

self-generating generator and a'reactance tube coupled to said selectivecircuit and responsive to said selected energy'and a control circuitcou, pling said detector to said reactance tube for controlling the meanfrequency of operation of said connections for deriving correspondingmodulated oscillations .from said generator, other means for derivingother oscillatory energy the frequency variations of which correspondsto modulation of the frequency of said generated oscillation, means forcombiningsaid two derived oscillations in degenerative sense to obtainoscillatory energy wherein the modulations have been reduced to phaselibrations Vwithin one radiari, a phase detector coupled to said lastnamed 4 means, and a control circuit coupling said phase the energy ofreduced deviation, a phase detector generator in a sense to nullifytendencies ofthe y mean frequency of operation thereof todrift,

7. In a frequency modulation system comprising a self-generatinggenerator and a reactance tube modulator for modulating the frequency ofthe generated oscillations in accordance with signals, a frequencystabilizing circuit for said generator including a .degenerative loop towhich said frequency modulated energy is fed and from which energycomprising primarily carrier and signal variations .representedby phasedeviations within one radian is selected.- a phasevmodulation detectorcoupled to said loop and responsive to said selected energy and acontrol circuit cou- Fling said detector 'to'said reactance tube forcontrolling the mean frequency of operation .of said generator in asense'l to nullify tendencies of themean frequency of operation thereofto drift.

8. In a timing modulation system'comprising a self-generating generatorand a modulator for modulating the timing -of the generated oscillationsin accordance with signals. a frequency stabilizing circuit for saidgenerator including ill degenerative loop to which said modulated energyis fed and from which energy comprising primarily carrier energy isselected, said degen-v erative loop including a converter wherein 'saidoscillations areheterodyned to alow frequelncy.

detector tosaid rst mentioned generator to change its frequency ofoperation in a sense to nullify saidv changes in its mean frequency ofoperation.

11. Inapparatus cillation generatorand modulator wherein the generatedoscillations are modulated through a wide range and wherein the meanfrequency of operation may change, connections for deriving oscillationscorrespondingly modulated and changed in mean frequency from-saidgenerator,

other means for deriving other 'oscillatory/energy the frequencymodulations of which corresponds to modulations in the frequency of saidgenerated oscillation, means for combining saidtwo derived oscillationsin degenerative sense to obtain oscillatory energy wherein themodulations have been reduced to phase librations with'one radian, avsource of oscillations of ,a iixed frequency equal to the meanfrequency of said last mentioned oscillatory energy, a phase detectorcoupled to said .last named means and to said last named source, and acontrol circuit coupling said phase detector to'said rst mentionedgenerator to change its frequency of operation in a 4sense to nullifysaid changes in its mean frequency of operation. t f

12. In apparatus ofthe class described, a frequency modulatedoscillation generator the mean frequency of operation of which maychange, connections for deriving. corresponding oscillations from saidgenerator, other means for'deriving other oscillatory energythe'frequency variations of the class described, an os-' of whichcorresponds to changes in the frequency' said other oscillations, and acontrol circuit coupling ,said phase detector -to said first mentionedgenerator to 'control its frequency of operation in accordance with thedirect current component obtained by said phase detection.

13. The method of compensating variations in the mean frequency oftiming modulated oscillations to obtain timing modulated oscillations ofsubstantially constant mean frequency which includes these. steps,heterodyning oscillatory enerey characteristic of said timing modulatedoscillations with other oscillatory energy characteristic of said timingmodulated oscillations in a. degenerative sense to derive resultantenergy the timing of which varies within small limits in a mannercorresponding to variations in the timing of said oscillations.detecting said resultant energy and varying the mean frequency of saidfirst mentioned timing modulated oscillations in a sense to oppose andnullify said first' variations in the mean frequency thereof. v 14. Themethod of compensating variations in the mean frequency of phase orfrequency varied oscillations to obtain oscillations of substantially ywithin the limits of about one radian in a manner corresponding to'variations in the phase or frequency of said first mentionedoscillations,.

detecting said phase variations of said resultant energy and varying themean frequency of said first mentioned oscillations in accordance withsaid detected phase variations and in a sense to oppose and nullify saidfirst mentioned variations in the mean frequency thereof.

15. In wave frequency stabilizing apparatus to be used to stabilize' themean frequency of wave energy the instantaneous frequency of which ismodulated in accordance with signals and the mean frequency of which maychange in an undesired manner, a current frequency converter,

means to feed to said converter currents of difjferent frequencies, theinstantaneous frequencies of which are modulated in a mannercorresponding to the modulations on the said wave energy and the .meanlfrequencies of which may changein said undesired manner, in'adegenerativesense such that the modulations on a, beat note resultingfrom mixing said currents in said converter are reduced to a small phaseamplitude, and means for selecting said beat note and opposing saidchanges in the mean frequency of said first mentioned wave energy inaccordance with the phase changes of the selected beat note.

16. In wave frequency stabilizing apparatus to be used to stabilize the.mean frequency of wave energy the instantaneous frequency of which ismodulated in accordance with si nais and the mean frequency of which maychange in an undesired manner, a` current frequency converter, meanscontrolled by said wave energy r to feed to said converter currents of afirst frequency the instantaneous frequency of which is modulated in amanner corresponding to the modulations on the said wave energy. meansccntrolled by said currents of said first frequency to 17. Apparatus asrecited in claim 16, wherein said first means includes a 'current mixerexcited by oscillations of ilxed frequency and by said first mentionedwave energy with a coupling between the mixer and converter, and whereinsaid second means includes an oscillation generator with a wavefrequency demodulator excited by l the selected beat note', a modulatorcoupling the demodulator tothe oscillation generator, and a couplingbetween the generator and the converter.

18. In a signalling system comprising a source of oscillations thetiming of which is modulated in accordance with signals'and the meanfrequency of which may drift, a circuit for stabilizing the meanlfrequency of said oscillations including a degenerative loop to whichsaid modulated energy is fed, and in which the timing l

